1. Field of the Invention
The present invention relates to a nitride-based semiconductor light-emitting device, and more particularly to a nitride-based semiconductor light-emitting device which employs a p-type clad layer structure with enhanced hole injection efficiency into an active layer, thereby exhibiting a high luminous efficiency.
2. Description of the Related Art
Generally, nitride-based semiconductor light-emitting devices are optical devices with a high output that generate short-wavelength light in the blue and green ranges and the like, and thus enable realization of the full color spectrum. For these reasons, nitride-based semiconductor light-emitting devices have drawn attention in related industrial fields.
Nitride-based semiconductor light-emitting devices are semiconductor single crystals composed of AlxInyGa(1-x-y)N (wherein 0≦x≦y, 0≦y≦1 and 0≦x+y≦1) which can be grown on substrates, e.g., sapphire and SiC substrates, by metal organic chemical vapor deposition (MOCVD).
Nitride-based semiconductor light-emitting devices essentially consist of an n-type clad layer, an undoped active layer and a p-type clad layer. A conventional nitride-based semiconductor light-emitting devices is sectionally shown in FIG. 1.
Referring to FIG. 1, the conventional nitride-based semiconductor light-emitting device 10 comprises a sapphire substrate 11, and an n-type clad layer 13, an undoped active layer 15 and a p-type clad layer 17 deposited in this order on the sapphire substrate 11. In addition, the light-emitting device 10 further comprises an n-side electrode 19a and a p-side electrode 19b which are connected to the n-type clad layer 13 and the p-type clad layer 17, respectively. The active layer 15 may have a multiple quantum well structure in which a plurality of GaN quantum barrier layers and a plurality of InGaN quantum well layers are alternately laminated.
When an electric current is applied to the electrodes 19a and 19b, electrons emitted from the n-type clad layer 13 and holes generated from the p-type clad layer 17 are recombined together in the active layer 15 having a multiple quantum well structure to emit short-wavelength light in the green or blue ranges.
As generally illustrated in FIG. 1, the p-type clad layer 17 includes an electron blocking layer (EBL) 17b formed on the active layer 15 and a contact layer 17a formed on the electron blocking layer 17b. The electron blocking layer 17b may be made of a nitride semiconductor containing Al, such as p-type AlGaN, whereas the contact layer 17a may be made of a nitride semiconductor containing no Al, such as GaN.
Since the AlGaN electron blocking layer 15b has a larger energy band gap than a nitride semiconductor containing no or a small amount of Al, it can effectively prevent overflow of electrons emitted from the n-type clad layer 13 without recombination with holes in the active layer. As such, the p-type clad layer 17 including the EBL 17b can decrease the number of overflowing electrons, contributing to an improvement in the luminous efficiency of the light-emitting device 10.
However, since AlGaN not only has a lower hole mobility than any other nitride semiconductor layers, but also has a relatively low hole concentration (about 1×1017/cm3), the injection efficiency of holes generated from the p-side electrode 19b to the active layer 15 may be lowered, thus causing a problem in obtaining a high luminous efficiency.
Thus, there is a need in the art for a novel nitride-based light-emitting device which maintains the advantage of an EBL, i.e., prevention of electron overflow, and at the same time, enhances the efficiency of holes injected into an active layer, thereby remarkably improving the overall luminous efficiency in a complementary manner.